2007 Blue Crab Stock Assessment

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A stock assessment for blue crab, Callinectes sapidus, in Florida waters

Michael D. Murphy,  Anne L. McMillen-Jackson, and Behzad Mahmoudi
Florida Fish and Wildlife Conservation Commission
Florida Marine Research Institute - St. Petersburg, Florida

Report to the Florida Fish and Wildlife Commission Division of Marine Fisheries 
June 2007

Executive Summary

In this second quantitative assessment for blue crabs in Florida we provide separate, detailed population analyses for blue crabs inhabiting waters along the gulf and Atlantic coasts. Although we treat these populations as separate, there may be a relatively high movement of blue crab from the gulf to the Atlantic population. Updated information is provided on key aspects of their life history (age, growth, and reproduction), habitat requirements, and population dynamics (genetic stock structure and natural mortality).

During the 1930's and 1940's, statewide blue crab landings were about 4.5 to 7.0 million pounds with about 1.0 million pounds reported on the gulf coast. The landings remained relatively constant through the 1950's and 1960's on the Atlantic coast, whereas gulf coast landings increased to about 7 millions pounds during the late 1950's then to 15 million pounds during the 1960's. Since 1965, the commercial landings of blue crab on both coasts have declined with the lowest landings reported occurring in 2001 (gulf) and 2002 (Atlantic). Through 2005, landings and commercial catch rates have rebounded somewhat. Landings were about 7.4 million pounds on the gulf coast and 4.2 million pounds on the Atlantic coast during 2005.

There are no estimates of the recreational harvest of blue crabs, making the conclusions of this assessment suspect if these landings are, as suspected, substantial.

Fishery-independent indices of abundance were variable at the start of these surveys (1989 or 1990) and showed a depression in abundance during the early 2000's on both coasts that mirrors the commercial catch rate trends. These abundance indices have rebounded through 2005.

All three assessment models indicate that fishing mortality rates have recently trended downward, since 2003 on the gulf coast and since about 2000 on the Atlantic coast. The analyses differ somewhat in the historical level of fishing mortality but all show a rapid increase in fishing prior to the 1960's then a general slow increase during the period 1960-1999 before the recent decline.

Estimated abundances of blue crabs appear to have responded to the recent declines in fishing mortality by increasing since the early 2000's. Historic trends in abundance show a general decline in biomass from the 1950's through the 1980's, a possible increase in abundance and biomass during the early to mid 1990's before a significant drop is seen during the late 1990's. All three analyses indicate a rebound in abundance in recent years.

The results of the three analyses point to somewhat different results for the status of the blue crab stocks in Florida. The catch-survey analysis indicated that overfishing was not occurring during 2002-2005, with respect to a rough estimate of the potential overfishing benchmark F0.1. The biomass dynamic model indicated that the fishery has been overfishing the stock on both coasts since the mid 1960's or early 1970's with respect to FMSY. This has apparently reduced the stock size below the biomass associated with maximum sustainable yield (MSY). Conversely, the stochastic stock reduction analysis shows that, though highly uncertain, the stocks on both coasts were most likely not being overfished during 2002-2005. A common feature in all of these analyses is the finding that blue crabs in Florida appears to be very resilient to high fishing rates. Estimates of MSY were consistent between these models at about 17.5 million pounds on the gulf coast and 7.5 million pounds on the Atlantic coast.

There are troubling gaps in our knowledge about blue crab and its fishery that could lead to a high degree of bias in these analyses. The ages of blue crab are not known and may lead to a biased estimate of natural mortality rate. There is some information to suggest that commercial discard mortality could be high and that the recreational catch could be significant. Neither of these is included in the analyses due to lack of any meaningful time series of these data.